This invention relates to a method and apparatus for proving fluid flow meters, such as gas meters and the like.
Gas meters presently employed by the gas utility industry are mechanical meters of the double bellows type. Such meters are proven using apparatus commonly referred to as a bell prover system. In such a system, a copper bell of accurate dimension is allowed to descend at a constant rate into a tank of light oil or water As the bell descends, a suitable test fluid, typically air or natural gas, is passed through the meter under test. The volume of air or natural gas which is passed through the meter is determined by the amount of linear movement of the bell. The position of the bell accurately defines the volume of test fluid which has been passed through the meter under test.
Typically, in proving gas meters using bell prover systems, a source of air or natural gas is connected to the meter under test and the flow rate of the gas meter is adjusted by selecting a suitable orifice which is connected in series with the source of test fluid and the meter under test. With the flow rate of the gas meter set, the proof run is initiated. With gas meters presently available, initiation of a proof run is effected by interrupting a light source using the calibration dial of the meter. At the start of the proof run, the test fluid supply is switched rapidly to the bell. After one cubic foot of the test fluid has passed through the meter of the test and the bell, the light source is interrupted because the calibration dial has registered one complete revolution. When the light source is again interrupted, the fluid outlet of the bell is closed off, terminating the supply of the test fluid to the meter.
The position of the bell is then accurately recorded electronically, yielding the exact amount of the test fluid that has passed through the meter under test during the time it recorded passage of one cubic foot of fluid as indicated by the calibration dial of the meter. From this measurement, the accuracy or proof of the meter can be calculated. The information obtained can be used to adjust the mechanical mechanism of a meter that fails the proof test.
In the co-pending U.S. patent application Ser. No. 140,714 of Pearman et al, which is entitled ELECTRONIC GAS METER, and which is assigned to the assignee of this application, there is disclosed a gas meter which includes a solid state sensor and solid state signal processing circuits for measuring gas flow volume. This gas meter does not have a calibration dial available for controlling a proof test in a manner similar to mechanical meters of the double bellows type as described above. Also, the meter does not have a mechanical adjustment to improve its accuracy. Thus, proof test techniques and calibration adjustments heretofore used for mechanical gas meters cannot be used on electronic gas meters of this type.
Moreover, the meter circuitry is powered from a battery. In order to conserve battery energy, a sampling technique is used by the gas meter, and the meter circuitry includes a timing function which defines meter operating cycles, the solid state sensor and associated processing circuits being energized only during a portion of each meter operating cycle for as much time as is necessary to maintain an accurate measurement of volumetric gas flow through the meter. The circuitry is energized during active periods or sampling intervals when flow measurements are conducted. The meter circuitry is held inactive for the balance of the meter operating cycle. The meter circuitry is not continuously measuring gas flow, but rather operates to average flow sample signals produced during successive operating cycles to provide flow measurement data. Accordingly, at any instant of time, the value of gas volume that has flowed through the meter, as recorded in a solid state memory of the meter, may be in error by the amount of time that the gas was flowing but that the meter system was maintained inactive. By way of example, this error may be as large as 1/18 of a cubic foot at any instant of time at the maximum flow rate. However, this error becomes zero each time that the meter becomes active and records the latest data.
Thus, it would be desirable to have a method and apparatus for proving a fluid flow meter of the type incorporating solid state sensing and signal processing circuits.
It would also be desirable to have a method and apparatus for proving a fluid flow meter of the type which operates in active and sleep modes, and which method and apparatus substantially eliminates error in the measurement of fluid flow volume through the meter during a proving test operation.